1. Field of Invention
The present invention relates to ignition systems for internal combustion engines and more particularly to ignition systems capable of maintaining the efficiency of the ignition system at high temperatures and which are capable of controlling overheating of the ignition coil.
2. Prior Art
In the ignition system of an automobile, the voltage generated in the secondary winding of the ignition coil is proportional to the primary current I.sub.1 which flows continually in the primary side of the ignition coil. The primary current I.sub.1 is affected by several external factors. The following are among such external factors which particularly influence the lowering of the primary current I.sub.1 :
(1) an increase in the number of engine revolutions n;
(2) AN INCREASE IN THE PRIMARY RESISTANCE OF THE IGNITION COIL AS A RESULT OF A RISE IN THE TEMPERATURE OF THE IGNITION COIL; AND
(3) A DECREASE IN THE VOLTAGE OF THE BATTERY.
In order to improve the conditions mentioned in (1) and (2) above, generally an external resistor is inserted in series with the primary winding of the ignition coil. Such a circuit is shown in FIG. 1. In FIG. 1, electric current from battery 1 is supplied via external resistor 2 to the primary winding 3a of ignition coil 3. The primary winding 3a and the secondary winding 3b of the ignition coil 3 are coupled to each other at one end. Between the junction formed by one end of the primary winding 3a and one end of secondary winding 3b and ground is inserted an interrupter 4, such as a set of distributor points, and a condenser 5 coupled together in parallel. The interrupter 4 interrupts the current flowing ignition coil 3 in response to the speed of rotation of the engine. Interruption of the current by interrupter 4 causes high voltage to be generated in the secondary coil 3b of ignition coil 3. The aforementioned condenser 5 is employed to absorb the electric arc that is produced at both ends of the interruption of the current through primary winding 3a of ignition coil 3.
Four ignition spark plugs 6a-6d are coupled to the secondary coil winding 3b of ignition coil 3. The high voltage from secondary coil 3b is selectively applied to these ignition plugs 6a-6d by a distributor (not shown).
The value of external resistor 2 is set substantially equal to the direct current resistance R.sub.1 of the primary winding 3a of ignition coil 3. Thus, the primary resistance, as seen from the battery side, is apportioned approximately half to the direct current resistance R.sub.1 of the primary winding of ignition coil 3 and half to the value of the resistance of external resistor 2. For this reason, the Joule's heat that is generated in ignition coil 3 is cut in half. In other words, if the Joule's heat generated by the current flowing through ignition coil 3 could be withstood without an external resistor 2, it would be possible to increase the primary current in ignition coil 3.
In order to generate in the secondary winding 3b of ignition coil 3 the same high voltage as was produced without the external resistor 2, the number of windings of primary winding 3a must be decreased by prescribed number, but the direct current resistance of the primary coil 3a must be set such that it will have a resistance value equivalent to the resistance value of the external resistor 2. In this way, even if the temperature of ignition coil 3 rises, it is possible to minimize the decrease of the high voltage that is generated in the secondary winding 3b.
In some applications, the circuit structure described hereinabove is not necessarily a fixed circuit. For example, since at the time of starting an engine a greater amount of current is required than the current required during normal operation, a means for shorting the external resistor is provided. Accordingly, whenever a greater current is needed, such means is actuated to short the external resistor 2. However, this method of increasing current has several drawbacks. First of all, at high temperatures (over 80.degree. C), the direct current resistance value of the primary winding 3a increases resulting in a deterioration of the ignition efficiency. Furthermore, if the ignition key switch is left in the on position while the engine is stopped, the external resistor 2 is shorted and the primary current in the ignition coil 3 continues to flow such that the temperature of ignition coil 3 rises abnormally resulting in a failure of the ignition coil.